The present disclosure relates to systems and methods for making direct reduced iron. Direct reduced iron (DRI) is a commercial product of more than 80% metallic iron, and, typically, more than 90% metallic iron, widely used as a source material for making steel. The remainder of the DRI product is gangue, which is high in silica. The conventional techniques for making steel involve the use of an electric arc furnace (EAF) or a basic oxygen furnace (BOF). DRI is typically higher in iron units than taconite pellets and other sources of iron and can be used as a partial substitute for scrap in production of steel by EAF.
DRI may be formed from beneficiated iron ore, such as taconite pellets. For example, taconite has been mined and crushed, and the iron containing portions magnetically separated from the non-magnetic portions to form a beneficiated product substantially higher in iron content than mined taconite. The beneficiated iron ore portion may be formed into pellets by pelletizing, and heated in the presence of a gaseous or solid reducing agent, such as natural gas, propane or coal, to a temperature below the melting point of iron to promote the reduction of iron ore to form DRI. The heating may be done in a linear hearth furnace (LHF) or in a shaft furnace such as the one disclosed in U.S. Pat. Nos. 3,749,386, 3,748,120 and 6,214,086. In any case, the reducing agent may be a gas percolated through the iron ore pellets or a solid such as coal mixed with the iron ore pellets before being heated in the furnace.
In any case, in the process to make DRI, the pelletized iron oxide containing material is moved through a furnace mixed with a reducing agent, such as natural gas, propane, coal, coke, or another form of carbonaceous material. A desulfurizing agent, such as limestone or dolomite, may also be added. The iron oxide material reacts chemically in the reducing zone of the furnace to partially reduce the iron oxide to form the DRI.
DRI is difficult to transport because it is highly reactive with oxygen in air and moisture. The DRI, known as sponge iron, has a high porosity with many voids making it porous in nature. With the high porosity of DRI, it has low compressive strength. When the DRI is stored, for example, in the hold of a ship or barge during transportation, some of the pellets have been prone to break apart under the weight of pellets above them, further promoting the likelihood of the DRI reacting with oxygen and moisture around it. Additionally, the rough surface characteristics of the DRI pellets produce DRI fines having a high surface area which also promoted the likelihood of the DRI reacting with the oxygen and moisture around it. Such DRI fines typically are produced throughout the transportation and storage of the DRI product making it difficult to transport DRI over long distances and to store DRI for long periods.
DRI is usually made, transported, and stored in the form of DRI pellets, accompanied by DRI fines. The porous, low internal strength, and flakey nature of such DRI compacts work to increase the surface area of the DRI compact that is exposed to an oxidizing atmosphere, resulting in substantial and rapid oxidation (rusting). The amount of DRI fines is thus increased during transportation and storage of the DRI product before delivery to the steelmaking furnace. The reactions that occur during DRI oxidation produce heat and hydrogen, making DRI susceptible to overheating and combustion. Increases in temperature in containers storing DRI, in which air is free to circulate, can reach 1200° F. Such combustion causes fires in the holds of ships and barges during transportation of DRI, and even in the clam shell buckets of cranes when unloading DRI. These circumstances have substantially increased the cost of DRI product delivered to a steel plant because of the losses of iron units in handling and hazards encountered during transportation and storage. Due to the difficulties and risks associated with transporting DRI product, production of DRI has, with a few exceptions, been generally located near the steelmaking facilities and near the time of use in steelmaking, rather than in more economical locations and times.
Consequently, various techniques have been used in the past to passivate DRI to reduce the risks associated with its pyrophoric properties. Examples of such passivation chemistry used in the past include aqueous solutions of a water soluble alkali metal silicate coating, organic amine vapors, or aqueous solution of organic amines coating, petroleum wax with a solid polymer of an olefin having 2 to 4 carbon atoms coatings, water soluble stearates, and like water repellant additive coating, hydrated calcined limestone (lime dust) coating, polymerizing aliphatic 1-olefin of less than 6 carbon atoms with catalytic material coating, ferritic chromium containing alloy coating, naphthenic petroleum/glycerol monoester coating, varnish, and heated paraffin wax coating.
Natural triglycerides such as soybean oil, sunflower oil, coconut oil, cottonseed oil, and castor oil have been proposed as a nonaqueous foam for use as a dust suppressant with iron ore pellets. See WO 2006/010721. These dust suppressants are preferably applied in the presence of a surfactant such HCF-740, a mixture of florosurfactants and hydrocarbon solvents, HCF-730, a nonionic mixture of silane surfactants, HCF-720, a nonionic mixture of silane surfactants and fluorosurfactants, or HCF-710 a nonionic mixture of silane surfactants and sulfonic acids. Id at 4. Such surfactants have been proposed to be used in amounts of 0.2 to 5% by weight per weight of DRI dust or fines. While iron ores are generally relatively stable oxides, DRI pellets and DRI fines are very different with high porosity and high reactive surfaces, and, accordingly, DRI pellets and DRI fines in the presence of these natural triglycerides tend to break down the triglycerides producing high levels of CO, which is highly undesirable with transportation of DRI.
Moreover, such treatment techniques generally have involved separation of the DRI fine particles from the DRI pellets and discarding the separated fines as waste. This involved reduction of iron yield and loss of iron units because the DRI fines were fugitive and not recovered. A need has existed for reclaiming the iron units of DRI fines to improve the efficiency of the use of DRI in steelmaking. Further, there still remains, despite various attempts, a need for an economic and efficient way of reducing the pyrophobic risks in transportation and handling of DRI pellets. A strong, stable, and pyrophobic product would enable the safe transport and storage of DRI, dramatically increasing the usefulness and effectiveness of DRI in steelmaking.
Disclosed is a method of reclaiming and inhibiting activation of DRI that comprises the steps of: (a) forming a moving stream containing pellets and fines of DRI, and (b) applying on said moving stream a coating material having a melting point between 70 and 200° F. and comprising at least one antioxidant and at least one carboxylic material comprising at least one selected from the group consisting of coatable fatty acid and an esterified derivative thereof to form a coating on the pellets and fines and (c) causing the fines to adhere together and to the pellets to form a plurality of agglomerates of DRI which are typically more than 90% metallic iron.
The coating material may be heated before application to form a liquid or semi-liquid and applied to the moving stream of DRI pellets and DRI fines as they are a falling stream when the step of coating occurs. The coating material may be applied as a liquid or semi-liquid in different droplet sizes to facilitate coverage of the fines and pellets with the coating material. Alternatively, the coating material may be applied as a solid or semi-solid, such as in the form of shavings, or granules to the stream of the DRI pellets and the DRI fines, on a conveyor and heating the DRI pellet and DRI fines with the coating material to promote the coating of the DRI pellets and DRI fines with coating material.
In any case, the coating material may be applied at a rate of less than 0.2% by weight or may be applied at a rate between 0.005 and 0.45% by weight per ton of pellets and fines coated. In any case, the coating material may be selected from the group consisting of palm oil, coconut oil, combinations thereof, and ester derivatives thereof, include at least one antioxidant and may include carboxylic material more than 30% by weight monounsaturated acid, and/or more than 30% by weight oleic acid. The antioxidant in the coating material may be selected from the group consisting of at least one of butlylated hydrooxytoluene, carotenoid, phytosterol, squalene, vitamin E, tocopherols, tocotrienols, and mixtures thereof. The coating material may be applied at a rate between 0.2 and 2.0 gallons per ton of coated DRI coated, or between 0.4 and 1.0 gallon per ton of DRI coated.
Alternatively, disclosed is a method of reclaiming and inhibiting activation of DRI fines comprising the steps of: (a) applying a coating material to a pile of DRI pellets and DRI fines while the pellets and the fines are added to, or removed from the pile, where the coating material has a melting point between 70 and 200° F., and including at least an antioxidant and may include at least one carboxylic material and at least one selected from the group consisting of fatty acid carboxylic material and an esterifed derivative thereof, (b) causing the DRI fines to adhere together and, optionally, to the DRI pellets to form a plurality of DRI agglomerates, typically more than 90% metallic iron; and (c) moving the DRI agglomerates formed from coated pellets and coated fines to a facility for present or future use in making steel.
Here again, the coating material may be heated before application to form a liquid or semi-liquid before or during the step of coating, and may be with different droplet sizes to facilitate coverage of the DRI fines and DRI pellets before or during the step of coating. Alternatively, the coating material may be applied as a solid or semi-solid, such as in the form of shavings or granules, to the DRI pellets and the DRI fines, and providing heat to promote coating of the DRI pellets and DRI fines. In any case, the coating material may be applied at a rate less than 0.2% of the coated DRI or may be applied at between 0.005 and 0.45% by weight per ton of DRI pellets and DRI fines coated. In any case, the coating material may be again selected from the group consisting of palm oil, coconut oil, combinations thereof, and ester derivatives thereof, to include at least one antioxidant and, in addition or in the alternative, may include carboxylic material more than 30% by weight monounsaturated acid, and/or more than 30% by weight oleic acid. The antioxidant in the coating material may also be selected from the group consisting of at least one of butllated hydroxytoluene, carotenoid, phytosterol, squalene, vitamin E, tocopherols, tocotrienols, and mixtures thereof. The coating material may be applied at a rate between 0.2 and 2.0 gallons per ton of coated DRI, or between 0.4 and 1.0 gallon per ton of DRI coated.
In another alternative, disclosed is a method of reclaiming and inhibiting activation of metallic iron fines comprising the steps of: (a) coating DRI fines with a coating material having a melting point between 70 and 200° F., including at least one antioxidant, and, alternatively or in addition, at least one selected for the group consisting of coatable fatty acid and an esterified derivative thereof to form a coating on the fines, and (b) causing coated DRI fines to adhere together to form a plurality of DRI agglomerates. The DRI agglomerates containing coated fines may then be moved to a facility for immediate or future use in making steel.
Again, the coating material may be heated before application to form a liquid or semi-liquid before or during the step of coating, and may be with different droplet sizes to facilitate coverage of both DRI pellets and DRI fines before or during the step of coating. Alternatively, the coating material may be applied as a solid or semi-solid, such as in the form of shavings or granules, to the DRI pellets and the DRI fines and heated to facilitate coating. In any case, the coating material may be applied at a rate less than 0.2% or may be applied at a rate between 0.005 and 0.45% by weight per ton of DRI pellets and DRI fines coated. In any case, the coating material may be applied at a rate less than 0.2% of DRI coated or may be applied at a rate between 0.005 and 0.45% by weight per ton of pellets and fines coated. In any case, the coating material may be again selected from the group consisting of palm oil, coconut oil, combinations thereof, and ester derivatives thereof, and, alternatively or in addition, the coating material includes carboxylic material more than 30% by weight monounsaturated acid, and/or more than 30% by weight oleic acid. The antioxidant in the coating material may also be selected from the group consisting of at least one of butylated hydroxytoluene, carotenoid, phytosterol, squalene, vitamin E, tocopherols, tocotrienols, and mixtures thereof. The coating material may be applied at a rate between 0.2 and 2.0 gallons per ton of coated DRI coated, or between 0.4 and 1.0 gallon per ton of DRI coated.
In yet another alternative, disclosed is a method of reclaiming and inhibiting activation of metallic iron fines comprising the steps of: (a) applying on DRI fines including metallic iron and iron oxides a coating material optionally having a melting point between 70 and 200° F., including at least one antioxidant and including at least one carboxylic material comprising at least one selected from the group consisting of coatable fatty acid and an esterified derivative thereof, and (b) by application and, optionally, agitation of the coating material on the DRI fines causing the fines to adhere together to form a plurality of DRI agglomerates.
In this alternative, the coating material may be heated before application to form a liquid or semi-liquid before or during the step of coating, and may be with different droplet sizes to facilitate coverage of a range of sizes before or during the step of coating. Alternatively, the coating material may be applied as a solid or semi-solid, such as in the form of shavings or granules, to the DRI fines. In any case, the coating material may be applied at a rate less than 0.2% b by weight of DRI coated or may be applied at between 0.005 and 0.45% by weight per ton of pellets and fines coated. In any case, the coating material may be again selected from the group consisting of palm oil, coconut oil, combinations thereof, and ester derivatives thereof, including at least one antioxidant, and, alternatively or in addition, including carboxylic material more than 30% by weight monounsaturated acid and/or more than 30% by weight oleic acid. The antioxidant in the coating material may also be selected from the group consisting of at least one of butylated hydroxytoluene, carotenoid, phytosterol, squalene, vitamin E, tocopherols, tocotrienols, and mixtures thereof. The coating material may be applied at a rate between 0.2 and 2.0 gallons per ton of DRI coated, or between 0.4 and 1.0 gallon per ton of DRI coated.
In describing these methods, DRI is used to describe traditional DRI products which have been identified as more than 80% metallic iron and are typically more than 90% metallic iron in commercial product (the remainder gangue), as well non-traditional materials such as baghouse dust and BOF dust which is high in iron units.